The present invention relates to optical systems using pulsed optical signals. More specifically, the present invention relates to generation and delay of optical pulse streams.
Optical beam control is often required where information from an optical beam must be relayed from one location to another. High-bandwidth, secure laser communication (e.g. pulse-burst encoding, pulse position modulation, etc.), infrared countermeasures (IRCM), target designation, bio/chem beam steering and laser radar are a few of the applications in which optical beam control is required. Optical beam control of pulsed optical beams requires that the control device provide time-coincident generation of the desired pulse format across the entire aperture of the control device.
Devices for steering optical beams are well known in the art. Optical beam steering can be implemented with electro-mechanical systems. Such systems generally consist of a mirror mounted on an electrical actuator. These systems provide relatively low losses for the strength of the reflected beam. However, such electro-mechanical systems are limited to low response frequencies up to the order of 1 KHz. The moving parts of an electro-mechanical system along with size and weight factors are considered to be major limitations of such a system.
Smaller and lighter optical beam steerers are provided by compact arrays of non-mechanical beam deflectors, such as optical MEMS mirrors (O-MEMS) or liquid crystal arrays. The optical signal provided to these devices is generally split into multiple optical signals. The arrays then actually consist of multiple optical radiators which act to steer and radiate multiple optical signals in a desired direction. However, since the radiators are generally deployed in a relatively flat plane, the output beams do not arrive at a receive point at the same time. This problem is particularly seen when the optical signal comprises pulsed signals. In this case, the optical pulse received from the radiating element furthest from the receive point will lag the pulse received from the closest radiating element. This problem is further exacerbated when the pulse widths (or the time slots for encoding) are shorter than the photon transit time across the radiating aperture. Performance of the optical transmitting system is improved when the individual optical beams are made time-coincident to create a time-coincident optical beam.
Applying a time delay to each optical beam before it is radiated provides the capability to generate a time-coincident optical beam. Controlling the delay of signals from individual transmitting elements is actually the principle behind a beamsteered phased array antenna system. Phased array antenna systems employ a plurality of individual antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. In a phased array, the relative phases of the signals provided to the individual elements of the array are controlled to produce an equiphase beam front in the desired pointing direction of the antenna beam. The premise of a true-time delay beamsteered phased array is to steer the array beam by introducing known time delays into the signals transmitted by the individual antenna elements. Accurate beam steering of a pulsed optical beam from individual optical elements similarly requires introducing time delays into the optical signals from individual optical elements to produce an equiphase optical beam front.
Optical control systems for producing selected time delays in signals for phased array antennas are well known in the art. Different types of optical architectures have been proposed to process optical signals to generate selected delays, such as routing the optical signals through optical fiber segments of different lengths; using deformable mirrors to physically change the distance light travels along a reflected path before transmission; and utilizing free space propagation based delay lines, which architecture typically incorporates polarizing beam splitters and prisms. These techniques can also be used for optical beam steering, with various levels of success.
The use of optical fiber segments to introduce delays requires the use of many optical switches and the splicing together of numerous segments of fiber. The costs of construction of such a device are substantial, given the significant amount of design work and precision assembly work required to produce a device having the range and incremental steps of time delays necessary to provide the desired steering. The numerous switching and coupling elements also introduce very high optical losses in the beamforming circuitry, requiring significant optical power.
The deformable mirror system relies on the physical displacement of a mirror to provide the necessary time delay; an array of moveable mirrors allows the generation of a range of delayed optical signals. This type of system introduces additional complexity into an optical beam steering system due to the tight tolerances and small time delays required for optical signals.
An optical architecture for time delay beamforming using free space elements is disclosed by Riza in U.S. Pat. No. 5,117,239, xe2x80x9cReversible Time Delay Beamforming Optical Architecture for Phased-Array Antenna,xe2x80x9d dated May 26, 1992. In Riza, input optical beams are directed through a plurality of free space delay devices which selectively delay the beams. The delay imparted to an individual beam is selected by a plurality of spatial light modulators coupled with polarizing beam splitters which will either pass a light beam or direct the light beam into a delay device. This architecture also requires a large number of individual delay devices, which increases the complexity and cost of the system.
An optical true-time delay bulk structure is disclosed by Zhenhai Fu and Ray T. Chen in xe2x80x9cFive-bit substrate guided wave true-time delay module working up to 2.4 THz with a packing density of 2.5 lines/cm2 for phased array antenna applications,xe2x80x9d Optical Engineering, Vol. 37, No. 6, June 1998, pp. 1838-1844. The bulk substrate disclosed by Fu and Chen comprises a passive waveguide that takes as an input an optical pulse and generates a sequence of output pulses with fixed delays. In this prior art, the passive substrate is used to provide delays to an optical signal and a photonic switching network is used to select a given set of taps. Holographic gratings are used to provide the output taps along the delay line. To assure that each tap has the same optical output power, the diffraction efficiency of the gratings is designed to increase along the delay line, as the successive taps couple the light out. Since the waveguide is passive, i.e., no external control is used to modify the delay provided by the waveguide, and, further, the tapped output locations are fixed, the output sequence of optical pulses is fixed in a temporal sense and cannot be changed. The device disclosed by Fu and Chen is directed to optically controlling an RF pulse-forming network with a fixed set of time delays.
Thus, it would be desirable to provide a mechanism for producing variable true time delay in an optical signal without requiring active switching and without high insertion loss. This mechanism would then allow for precision optical beam steering. In addition, it would be desirable to provide such a true time delay which is relatively simple, compact, and inexpensive.
It is therefore an object of the present invention to provide a device and method for providing true-time delayed optical signals without requiring active switching or incurring high insertion loss, such that the device and method can provide multiple delayed optical signals for input to an optical beam steering array.
It is a further object of the present invention to provide the desired delayed optical signals in a relatively simple, compact, and low cost manner.
Pursuant to the present invention, a method and apparatus is provided which produces several optical pulse streams with controllable time delays between the different pulse streams. The controllably-delayed optical pulse streams may be used to control a beam steering system so that the composite output beam exits the overall aperture of the beam steering system at the same time, regardless of the output angle. The beam steering system may be operated in a reciprocal fashion so that the beam steering system receives a composite optical beam at the same time, regardless of the input angle.
Multiple time delayed optical pulse streams are produced by coupling an optical pulse stream into a bulk or waveguide structure that contains multiple tapped output locations, similar to the structure disclosed by Fu and Chen. However, unlike the structure disclosed by Fu and Chen, the structure of the present invention comprises electro-optically active material for which the refractive index of the material changes depending upon the voltage applied across the material. As is known in the art, changing the refractive index of a material in which an optical signal is being transmitted results in changing the speed at which the signal is transmitted through the material, and thus provides for imparting a controllable delay to the signal.
A single controllable voltage may be uniformly applied across the structure or several separately controllable voltages may be applied at different locations across the structure. When a single voltage is applied to the structure, the delay between the output optical pulse streams from the output taps will change in proportion to the change in the applied voltage. If the structure is configured such that the delay between the pulse streams from each of the output taps is the same, a change in the applied voltage will change the delays between all of the pulse streams by the same temporal amount. If separately controlled voltages are applied at multiple locations across the structure, the delay of the pulse stream from each output tap or groups of optical taps can be separately controlled or changed as needed. Groups of output taps can be controlled so that pulse streams output by the structure are partitioned into smaller subsets of pulse streams. If a different voltage is applied at each output tap and the different voltages are varied on a pulse-by-pulse basis, each pulse in each pulse stream may have a unique temporal spacing relative to all of the other pulses output from the structure.
In a first embodiment, the present invention comprises a bulk or optical substrate delay line having a specified thickness. The delay line consists of a series of output ports, where the optical propagation delay at the output ports is electro-optically controllable. The output ports can be in the form of a series of independent gratings that diffract the incident optical beam at preferred locations along the waveguide. An optical pulse stream is coupled into the delay line at one end and reflects between internally reflective surfaces of the delay line. Since, the optical pulses travel from one surface to the other, the delay between the output ports is proportional to both the distance between the ports and the thickness of the delay line. The device can be viewed as a tapped delay line whose taps are all equally spaced in space (and, therefore, in time) along the waveguide. Preferably, the tap-to-tap temporal delays along the length of the waveguide are all the same, resulting in a series of parallel optical outputs with increasing delays. The tapped delays may all be controlled by a single voltage (via the electro-optic effect), applied across the device. Alternatively, the device can be electronically partitioned so that different subsets of taps can possess different, but, controllable, time delays. In this case, different control voltages are applied at different locations along the guide, resulting in subsets of the parallel outputs, each with its own delay sequence.
In a second embodiment, the present invention comprises a planar waveguide electro-optically active structure positioned within a cladding substrate. Transparent electrodes and gratings are contained in the cladding substrate. An incident optical beam is coupled into one end of the structure and propagates in a direction substantially parallel to the cladding substrate. The incident optical beam is diffracted out of the wave guide structure by the gratings to provide multiple delayed optical beams, where the delay between the beams is a factor of the spatial displacement of the gratings and of a voltage applied to the electrodes across the electro-optically active structure. The electrodes may be partitioned into several regions, providing the capability for separately controlling the delay applied to separate sets of beams.
An optical beam steering system with true-time delay characteristics is provided in accordance with the present invention. The optical outputs from the tapped delay lines can be directed into a device for compensating for the fixed delays provided by the tapped delay line. The outputs may also be directed into an array of optical phase-shifting elements, with each phase-shifting element operating on a separate output. These outputs can then be directed into a discrete set of beam-steering elements, such as an optical micro electro-mechanical system set of mirrors or a liquid crystal phased array. The combination of true-time delay and phase compensation provides that the beam leaving the array will emerge with a uniform phase front. This will allow the diffraction of the output beam to be dictated by the scale size of the entire aperture, rather than that of a sub-aperture or discrete element.
The benefit of such a beam steering system is that very short pulses can be steered into a set of given directions, with the initial short pulse-width maintained and its spatial output diffraction limited (at least at the transmitter aperture). By electronically partitioning the tapped optical delay line, a set of beams can be simultaneously directed into several different directions, so that an optical distribution network can be realized, with each link maintaining temporal and spatial coherence (i.e., true-time delay and spatial phasing). The basic system can enable high-bandwidth information to be distributed to a single location or to several different locations in space (simultaneously) from a common transmitter.